This application claims the priority of German patent document 199 44 536.2, filed Sep. 17, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for periodically reactivating copper-containing catalyst material which is introduced into a reactor for the catalytic conversion of a hydrocarbon or hydrocarbon derivative starting material by an oxidation and/or a reforming conversion reaction and is gradually deactivated by the conversion reaction.
Copper-containing catalyst materials are used, for example, in reactors which are used in fuel cell vehicles for obtaining hydrogen through the catalytic conversion of methanol or the like by a partial oxidation reaction and/or a steam reforming reaction. It is known that the catalyst material is gradually deactivated by the methanol conversion reaction. Various methods for the periodic reactivation of the catalyst material are known to enable it to substantially regain its original activity.
One of these known reactivation techniques comprises applying an oxygen-containing gas stream, e.g. air, to the deactivated catalyst material. It is specifically proposed in DE-B1 246 688 for normal reactor operation, during which steam reforming of methanol is carried out at a temperature of from 150xc2x0 C. to 400xc2x0 C. in the reactor, to be periodically interrupted and for the catalyst material to be regenerated (i.e. reactivated) through the application of the oxygen-containing gas stream at a temperature in the range from 150xc2x0 C. to 450xc2x0 C. A hydrogen-containing gas stream is then applied to the catalyst material before normal methanol conversion operation continues. In publication JP 4-200642 (A), it is proposed to apply an oxygen-containing gas stream, which contains at most 5 mol % molecular oxygen, at a temperature of between 120xc2x0 C. and 650xc2x0 C., to reactivate the copper-containing catalyst material.
U.S. Pat. No. 4,855,267 discloses a two-stage reactivation method in which an oxygen-containing gas stream which contains at least 1 vol % of oxygen is initially applied to a deactivated, copper-containing catalyst material at a temperature of between 275xc2x0 C. and 400xc2x0 C., for a period of approximately two hours to eighteen hours. The catalyst material which has been oxidised in this way is then subjected to a reducing treatment in a reducing atmosphere, at a temperature of between 100xc2x0 C. and 400xc2x0 C., for a period of approximately two hours to three hours.
DE 1246688 discloses a method for the catalytic steam reforming of methanol. In reaction phases that are periodically interrupted by regeneration phases, the starting steam mixture is catalytically converted at 150xc2x0 C. to 400xc2x0 C., preferably at 240xc2x0 C. to 270xc2x0 C. In the regeneration phases, the catalyst system, which preferably comprises a nickel catalyst and a zinc-/copper- containing catalyst in separate reactors, is regenerated by treatment with an oxygen-containing gas at elevated temperatures, preferably at temperatures in the range from 150xc2x0 C. to 450xc2x0 C. Before the methanol conversion reaction is restarted, it is possible to pass a hydrogen-containing gas across the catalyst beds at elevated temperatures in order to reactivate the nickel catalyst.
DE-B-1094244 describes a method for the regeneration of a catalyst comprising copper and sodium silicate by oxidation of a carbon-containing residue which has been deposited thereon. A hot, oxidizing gas which contains less than 1.5 vol % molecular oxygen is passed over the catalyst, while substantially avoiding condensation of steam, at a temperature which is above 105xc2x0 C. but below the sintering temperature of the copper, which is dependent on the amount of sodium silicate. The regeneration operation is only ended when the copper has been virtually completely oxidized to form copper oxide. Then, the temperature is increased to a temperature which is above the sintering temperature of copper but below 425xc2x0 C. and the oxygen-containing gas continues to be passed through.
The present invention is based on the technical problem of providing a method in which a catalyst material that is gradually deactivated in normal operation can be reactivated relatively easily and effectively and which is also eminently suitable for mobile use, for example in fuel cell vehicles, without significantly disrupting driving operation these vehicles.
A method according to the present invention relates specifically to the warm operating state of the reactor containing the catalyst material. An oxygen-containing gas stream is applied to he catalyst material for the purpose of reactivating the catalyst material and is always interrupted at the latest when a monitored temperature of the catalyst material exceeds a maximum level which is more than a predeterminable tolerance level above an operating temperature. The operating temperature that is predetermined for the conversion reaction of the hydrocarbon or hydrocarbon derivative starting material. For example, a reactivation sequence comprises relatively short pulses of the oxygen-containing gas stream being applied to the catalyst material.
The catalyst material heats up as a result of the oxygen-containing gas stream due to exothermic oxidation processes (e.g., oxidation of the copper and/or deposits on the catalyst material). The catalyst material cools back down to below the predeterminable maximum temperature level in the periods between successive pulses of oxygen-containing gas stream. In this way, the catalyst material is prevented from overheating.
In another embodiment of the present invention, an air stream which is diluted with steam is used as the oxygen-containing gas stream for the reactivation of the copper-containing catalyst material. The dilution enables the duration of the individual application cycles of the oxygen-containing gas stream to be lengthened compared to the use of undiluted air or a gas stream with an even higher oxygen content, without the temperature of the catalyst material rising beyond the predetermined maximum level. If appropriate, the dilution may be so great that the entire reactivation operation can be carried out without interrupting the application of the oxygen-containing gas stream to the catalyst material.
Steam may also be used as an intermediate purge media before and/or after the application of the oxygen-containing gas stream. In this way, it is possible to prevent oxygen and the starting material which is to be catalytically converted from coming together in exothermic stoichiometric proportions which, as a result of oxidation of starting material, would lead to even more intensive heating of the catalyst material. Particularly in the case of mobile use in fuel cell vehicles, the use of water as a diluting medium or intermediate purge medium has the advantage of it already being present for the steam reforming reaction of the starting material, such as for example methanol.
When a reactor is used in a vehicle, the reactivation is preferably started when a refuelling operation is detected. Since a refuelling operation requires a predictable time, there is no need to worry about the reactivation interfering with normal operation. Alternatively, of course the reactivation can be initiated by the driver.
The method according to the present invention may also be used for cold-start phases of the reactor. In this case, a mixture of a fuel and an oxygen-containing gas stream is initially applied to the catalyst material, resulting in exothermic oxidation of the fuel and, at the same time, oxidation of the catalyst and/or deposition on the catalyst. As a result of the two exothermic processes, the catalyst material relatively quickly reaches a temperature at which the catalyst oxidation and/or the oxidation of the deposits can take place with sufficient efficiency, after which the supply of the fuel is ended and only the oxygen-containing gas stream is applied to the catalyst material. The temperature at which the change to only the oxygen-containing gas stream is supplied is typically significantly below the operating temperature level for the normal catalytic conversion reaction of the starting material. The supply of the oxygen-containing gas stream is ended at the latest when the monitored temperature of the catalyst material exceeds the predeterminable maximum level, provided that the reactivation operation has not already been terminated as a result of the catalyst material having regained its fresh, reactivated state.
In another embodiment of the present invention, the oxygen content in the mixture which is initially supplied to the catalyst material is set at significantly superstoichiometric proportions. Furthermore, it may be advantageous for the stoichiometry of the gas stream not to be suddenly switched over from a predetermined level to zero, but rather for the stoichiometry to be changed as early as during the cold-start phase. In this case, the ratio between the mass flow rate of fuel FB and the mass flow rate of oxygen FO. may range from substoichiometric combustion (FB/FO greater than 2/3 with methanol as the fuel) to a fuel-free gas mixture (FB/FO=0) and may be set variably during start-up.
In a further embodiment of the reactivation method for reactor cold-start phases, the temperature switch-over level for switching over to supplying only the oxygen-containing gas stream is in the range between 100xc2x0 C. and 200xc2x0 C., preferably in the region of 150xc2x0 C. This temperature is still significantly below the typical operating temperature level of approximately 200xc2x0 C. to 350xc2x0 C. for a steam reforming of methanol.
Further, when the method starts, it is additionally possible to record the temperature of the catalyst material and to compare it with a second, lower threshold. When the temperature exceeds this second threshold, the method is started directly through the addition of the oxygen-rich gas stream. This delineation between a cold start and a warm start makes it possible to save fuel.
Another embodiment of the present invention keeps a reactor warm after operation has ended. In this case, the temperature of the catalyst material is continually monitored. Whenever the temperature falls below a third, lower threshold, an oxygen-containing gas stream is applied to the catalyst material until the monitored temperature of the catalyst material exceeds the predeterminable maximum level. The third, lower threshold is selected such that spontaneous oxidation with the oxygen-containing gas stream is ensured. With this method, it is possible to keep the reactor warm during prolonged pauses without consuming fuel.
The tolerance level by which the maximum temperature of the catalyst material may exceed the normal temperature level of the fuel conversion during the reactivation is fixed to at most approximately 100 K, preferably at most approximately 20 K. This allows the normal operating temperature of the catalyst material to be exceeded for short times without damage while it is being reactivated.
Advantageous ways of realizing the method according to the present invention are described below by way of example and to represent further ways which will become obvious to the person skilled in the art employing the teaching according to the invention. The examples of the method can be used in particular for periodic reactivation of a copper-containing catalyst material which is situated in a reforming reactor of a fuel cell vehicle. Mobile reforming reactors of this type are used to obtain the hydrogen which is required for the fuel cells from methanol or another hydrocarbon or hydrocarbon derivative fuel, with the participation of steam, by an oxidation and/or reforming reaction.